1. Field of the Invention
The invention relates to an apparatus and methodology for thermally mediated therapeutic procedures, and in particular for selective coagulation of specific components in selected regions in a biological matrix, when using focused ultrasound or electromagnetic radiation, such as a laser or microwave radiation, to induce thermal injury to the tissue.
2. Description of the Prior Art
Various thermally mediated therapeutic procedures using microwave, infrared or visible light energy have been used to induce coagulation necrosis of certain tissue components while protecting superficial tissues from thermal injury. For example, successful laser treatment of dermatoses, such as port wine stain lesions, hemangiomas, and telangiectasias, is based on photocoagulation of blood vessels without inducing thermal injury to the overlaying epidermis and papillary dermis, which could result in skin-surface texture changing or scaring.
Cryogen spray cooling is a potentially effective method for protecting superficial tissues from thermal injury. By the application of a short duration spurt of cryogen in the order of milliseconds, rapid and selective cooling of tissues is possible. Superficial tissues are cooled while the temperature of deeper tissues remain unchanged. Evaporation of the cryogen on the surface provides a mechanism for rapid removal of heat from the tissue. For example, when tetrafluoroethane, having a boiling point of -26.degree. C., is used as a cryogen, surface temperature drops of the order of 30-40.degree. C. have been obtained within 5-100 milliseconds. Experimental results indicate that spray cooling the tissue, e.g. skin, with tetrafluoroethane just prior to laser irradiation can selectively cool the superficial layer, e.g. epidermis, and yet permit photocoagulation of deeper dilated structures, e.g. port wine stain blood vessels. Successful blanching of port wine stain lesions without either epidermal thermal injury or skin-surface textural changes have been reported when the skin is precooled with tetrafluoroethane immediately prior to flashlamp-pumped pulsed dye laser irradiation with a relatively high-light dosage of about 10 J/cm.sup.2.
Surface cooling with ice or water has also been used as a method to prevent laser-induced thermal injury to the epidermis. Although this method has been shown effective in cooling skin, exposure time are too long (seconds) to reduce the temperature of the underlying targeted blood vessels. Consequently, incident laser energy is utilized ineffectively by first rewarming the blood vessels to their higher initial temperature before sufficient heat is generated to induce photocoagulation.
Some investigators have studied the effectiveness of delivering a short cryogen spurt, on the order of milliseconds, to selectively cool the epidermis without reducing temperature of the underlying targeted blood vessels. When used in conjunction with flashlamp pumped pulsed dye laser irradiation for treatment of vascular lesions, use of cryogen spray cooling has been demonstrated to prevent skin textural changes that result from thermally induced epidermal injury, while allowing blanching of port wine stains. In the absence of cryogen spray cooling, epidermal necrosis and subsequent skin pigmentation changes have been observed when treatments are administered with equivalent laser irradiation parameters.
The foregoing treatments have all been used for superficial dermal mediation, typically relating to hair removal, wrinkle treatment, and port wine stain removal. Laser Treatment without cooling has been practiced for tattoo removal. Hemangiomas are vascular tumors, characterized by rapid endothelial cell proliferation, which may infiltrate the entire dermis and extend several millimeters in depth. The depth and size of hemangiomas are far deeper than chromophores targeted for hair removal, wrinkle treatment, and port wine stain. Although laser irradiation has been used to induce photothermal destruction of hemangiomas, thermal damage to the epidermis and papillary dermis remain a serious concern. Therefore, what is needed is an apparatus and methodology whereby deep photocoagulation without thermal damage to the overlying epidermis can be successfully practiced.